If the particular tube is faulty, I would stop using it altogether. Just replace that pair and see if the problem goes away. This would not make anything worse. If the problem persists with the replacement, stop the amp and call Steve.

One of my Ob3a's does something similar just not as often or obvious. Since I love them and it hasn't affected the sound negatively they've stayed in place. Pick up a couple back up pairs and tube swap to see if there is an audible change. I suggest NOS RCA's.